Typically, a semiconductor device is fabricated with repetitive thermal treatments such as thermal oxidation, thermal diffusion, and various annealing processes. An annealing process is widely utilized for recovering the crystallinity after impurity ion injection, improving the contact characteristic of Al/Si and the interface characteristic of Si/SiO2, sintering for forming silicide, etc.
The thermal treatment is carried out with a rapid thermal processing (RTP) apparatus together with a furnace. The RTP apparatus can achieve an expected high temperature and minimize harmful impurity diffusion in a short processing time (from a few seconds to a few minutes). Thus, the RTP apparatus is widely used in thermal treatment processes.
A conventional rapid thermal processing will be described hereinafter with reference to the accompanying drawings.
FIG. 1 is a cross-sectional view of a wafer which illustrates the shortcomings caused by a conventional thermal treatment process. FIG. 2 is a graph illustrating states of wafers processed by the conventional thermal treatment process.
Referring to the drawings, a wafer on which a transistor is held in a cassette. The illustrated transistor has a source, a drain, and a gate. A metal thin film is deposited on the transistor by means of sputtering.
The cassette holding the wafer is mounted on a support plate. The support plate separates a load lock chamber and a shuttle chamber.
Next, the load lock chamber is vacuumized by pumping out the oxygen by means of a pump installed inside an exhaust line of the load lock chamber. After the load lock chamber reaches a vacuum state, nitrogen is injected into the load lock chamber through a nitrogen injection line so as to keep the load lock chamber at a pressure higher than the atmospheric pressure.
While maintaining this pressure, a robot sequentially loads wafers, each of which is held in the cassette inside the load lock chamber, on a susceptor of the process chamber. The wafers are thermally treated in the nitrogen gas atmosphere.
Next, each thermally treated wafer is taken out of the process chamber, cooled on a cooling station (not shown) of the load lock chamber, and then held in the cassette. All of the wafers are thermal treated in this manner.
However, if over a predetermined level of oxygen remains inside the load lock chamber after the pumping out process for vacuumizing the load lock chamber, the remained oxygen may diffuse into the process chamber when a wafer is moved from the load lock chamber to the process chamber.
As shown in FIG. 1, if the thermal treating process is carried out to form a silicide in this oxygen-including environment, the thin metal film (110) is so oxidized as to form a metal oxide layer (120) since the thin metal film reacts with the oxygen at a much faster speed than it reacts with the silicon layer (130) at their interface. Further, the entire metal thin film can be oxidized and transformed into a dielectric layer.
Also, the oxidization of the thin metal film increases the contact moat resistance so as to cause malfunction of the device, thereby resulting in degradation of the reliability of the device.
The wafers, (i.e., the third to tenth wafers) loaded after oxygen is diffused inside the process chamber react as described above such that the surface of the thin metal film is oxidized, resulting in increase of the contact moat resistance. However, the wafers following the tenth wafer may have a normal contact moat resistance through the normal thermal treatment process since the diffused oxygen will have been consumed with the previous wafers. Also, as shown in FIG. 2, when the wafers held in the cassettes are sequentially thermally treated with the conventional technique, the first and second wafers can exhibit normal contact resistance since these wafers have been processed before the oxygen diffuses from the load lock chamber to the process chamber.